Sp1 and other members of the Sp-family of mammalian transcription factors play a central role in the regulation of gene expression. At least eight distinct genes encode proteins that comprise this family, the majority of which are expressed in all tissues independent of cell proliferative index. Each of these eight genes carries a highly homologous DNA-binding domain consisting of three zinc-"fingers" and trans-activation domains that show varying degrees of homology between family members. Despite the close conservation of their DNA-binding domains, it is clear from studies of "knockout" mice that Sp-family members regulate only partially overlapping genes. Although such studies underline functional differences between Sp-family members, the cis-acting signals and trans-acting factors that dictate whether an Sp-dependent gene will be serviced by one or more members have yet to be identified. This proposal seeks to address these deficiencies in our understanding of Sp-mediated transcription by defining functional differences between Sp-binding sites and Sp proteins via the identification and characterization of consensus DNA-binding sites and components of the basal transcription complex that govern Sp-mediated transcription. As a consequence of our previous studies of Sp-family members we discovered that one member, Sp3, encodes three functionally distinct proteins, including two internally initiated isoforms that repress Sp1/Sp3- mediated transcription. Since a variety of data indicate that many Sp-dependent genes that are regulated by extracellular stimuli are bound by Sp1 and Sp3, we hypothesize that understanding the regulation of Sp3- mediated repression may shed light on the control of Sp1/Sp3-dependent promoters by extracellular stimuli. Additional goals of this proposal are to test this hypothesis in vivo and to isolate and characterize genes that may be regulated preferentially by particular Sp-family members. In so doing, we will link specific Sp-family members and their regulators with extracellular signals that govern the synthesis of Sp-dependent genes.